Ribosomes (or subunits) are released on chain termination. Dissociation factor insures maintenance of a pool of subunits, which subsequently react with specific initiator aminoacyl-tRNA, initiation factors and GTP to translate the initiation codon on mRNA. Studies in this laboratory have revealed in rat liver supernatant a binding factor specific for 40S subunits and natural (Met-tRNAf) or artificial (acetylphenylalanyl-tRNA) initiator tRNAs, and a supernatant protein which inhibits the interaction between 40S subunits and Met-tRNAf (but no Met-tRNAm or acetylphenylalanyl-tRNA); inhibition is due to hydrolysis of 40S-bound Met-tRNAf. The soluble Met-tRNAf binding factor and the Met-tRNAf hydrolase have been extensively purified and characterized. Recent studies with native ribosomal subunits obtained from rat liver revealed three activities associated with native 40S subunits. One of the activities catalyzes binding of acetylphenylalanyl-tRNA; another prevents binding of Met-tRNAf and inhibits the binding of Met-tRNAf promoted by soluble binding factor; the third, causes dissociation of 80S ribosomes and prevents reassociation of ribosomal subunits. The three activities have been identified in extracts obtained from native 40S subunits the characteristics of the reactions have been established, and assays for their quantitative determination have been developed. Some resolution and purification of the three 40S-associated factors have also been obtained. Attempts to obtain the proteins in homogeneous form, and to examine the mechanism of action of each, are in progress. The role of the factors in the formation of intermediates in the ribosoaml cycle and in the initiation of protein synthesis will be investigated. Comparison of the structural features of the factors from native subunits, and the activities that they catalyze, with those obtained from the supernatant fraction, will be carried out.